Azaleas indoors

The many cultivars of Rhododendron simsii (often called Rhododendron indicum) are popular pot plants which are brought on, or ‘forced’, in warm, humid conditions for sale at Christmas.

Indoor azaleas

Quick facts

Common name Indoor azalea
Botanical name Rhododendron simsii
Group Evergreen
Flowering time Winter
Height and spread 45-60cm (18in-2ft)
Aspect Light, dappled shade
Hardiness Frost tender
Difficulty Difficult

Cultivation notes

Unlike outdoor azaleas and rhododendrons, indoor azaleas are not frost hardy and are therefore not suitable, in frost-prone areas, for permanently planting in the garden when their period of flower is over. However, they can be kept and grown on as pot plants for flowering in future years.

Growing indoor azaleas as houseplants

Keep flowering plants in a cool, humid atmosphere with plenty of light, such as an unheated porch. Plants will deteriorate if they suffer long periods in hot, dry conditions.

After flowering

Remove the dead flowers. Repotting can then be done if necessary, increasing the pot size by no more than 2.5-5cm (1-2in) in diameter. Most indoor azaleas are imported from the Continent. They are usually growing in a pine needle, peat and leafmould medium, but a suitable alternative medium can be made from equal parts leafmould, peat substitute such as composted bark, and lime-free sharp sand. Acidic, partially decayed pine needles can be included if available; the essential point is that no alkaline materials should be used. A proprietary, ericaceous or lime-free compost is also suitable. Water-in using rainwater.

Where possible keep the plant in a warm greenhouse or conservatory, with a temperature of 13-16°C (55-61°F). Mist daily and water as needed to encourage new root and shoot growth. Alternatively, give the protection of a frost-proof cold frame or greenhouse, where growth initially may be slower.

For plants kept indoors, continue to house the plant in a cool, humid room until after the risk of frosts has passed in your area.

Summer treatment

Harden off  plants in May by transferring them from warm to cooler conditions and standing them outdoors when danger of frost is past. Plunge the pots to the rim in a bed of peat or peat substitute to avoid the danger of frequent drying out.

If plunging in soil, place some coarse, gritty material at the base to discourage worms from entering the pot through drainage holes. A cool, partially shaded position is desirable, but in deep shade flower buds may not form.

Flat dwellers should try to find a cool, moderately shady position in the home, or on a balcony, perhaps shaded by other plants such as climbers.

Bring under cover in September before any risk of frost, and place in a position of good light, avoiding direct, scorching midday sun. Ideally site in a cool, moist area of the greenhouse with night temperatures in the region of 7-10ºC (45-50ºF).

Watering and feeding

While plants are growing indoors, never allow the compost to dry out completely. If it does, soak in a bucket of water until no more air bubbles rise from the compost. Do not leave soaking for an excessive period of time as this can drown the roots.

Careful watering with rainwater is essential throughout the summer; if plants are left to suffer drought conditions, growth may be stunted and flower buds may fail to develop or may only partially develop, eventually falling unopened or shrivelling on the plant. Avoid using tap-water particularly if you live in a hard-water area. This contains dissolved sodium bicarbonate which is alkaline and is toxic to the plant.

Azaleas do not generally require extensive feeding, but when the first signs of new growth are seen, a general purpose proprietary liquid fertiliser may provide a useful stimulus. Apply at half the recommended rate for pot plants on two or three occasions up until the end of June. It is not necessary to feed in the first growing season after repotting.

Reflowering

There will be less demand for water as the weather gets cooler, but water requirements will increase again as buds swell and flowers open. Ventilate freely with rising day temperatures. Commercially, plants are brought into flower early by forcing in warm, humid conditions, with temperatures approximately 18-21ºC (65-70ºF) for six to eight weeks. Therefore, plants grown by the amateur gardener will be later to bloom.

A point to remember is that they will need to be grown on in a greenhouse if they are to bloom as profusely as when initially purchased. In addition, following forcing, it may be a year or two before they flower well, if a greenhouse or conservatory is not available.

Pruning and training

Longer shoots can be lightly pruned or shortened back after flowering – but not into older wood, which may not reshoot successfully.

Propagation

Softwood cuttings: Commercial propagation is by means of softwood cuttings taken in early spring and rooted under mist propagation. Cuttings need a good level of warmth at the base to root successfully. The amateur gardener is more likely to be successful  if using a small electric propagator with bottom heat.

Alternatively, try inserting four or five cuttings in a small pot covered with a polythene bag. Flimsy opaque polythene bags with one or two holes perforated in them are suitable as long as they transmit a good level of light, and cuttings under them may suffer less stress than under clear polythene.

Semi-ripe cuttings: Cuttings can also be taken when the young side growths are becoming firm towards the base, but are still relatively soft and supple at the tips. Try taking batches at 2-3 week intervals from early July until mid-September. This will ensure that some cuttings are taken at different stages of ripeness of the growth. Keep notes to refer to in future years on the best method for your growing conditions.

Problems

Flower buds not opening: In warm, dry atmospheres flower buds may have difficulty opening unless moistened by misting. Do not mist after the buds show colour.

Yellowing of leaves: This can occur where the growing medium has poor texture and remains sodden for a considerable time after watering. Yellowing may also occur due to iron deficiency resulting from the use of alkaline potting materials, watering with very hard tap water or over-feeding.

Leaf drop: Leaf drop can occur following a period of drought. The oldest leaves are the first to drop. Higher leaves may show browning at the leaf tip or edge. Leaf drop can also occur following extended periods of waterlogging as the impaired roots are not able to replenish water lost through the leaves.

As with all evergreen shrubs, each year a proportion of the foliage (mainly older leaves) is shed in spring and summer. This is normal and not a cause for concern.

See our page on general leaf problems in houseplants.


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